stm32f103 - minimize delay due to usart transfer

[Bogdan]

Posted on February 04, 2017 at 12:45

Hello all,

i am using a stm32f103c8t6 device, clocked at 72 MHZ,   the periph clock for adc is 12MHZ  ( 72/6).   

Using  adc injected function with  sample time configured  at ADC_SampleTime_1Cycles5

Timer 1 is generating a pwm wave on channel 4 ( CC4), and the cc4 signal is feed out trough TRGO and onto the ADC injection trigger input.

Till here everything works as expected, i have the correct voltage when reading the injection.

My ''without USART''  ADC interrupt handler.... looks like this ,   on the logic analyzer judging by the gpio toogle time, this routine runtime takes arround 1us to run.

void ADC1_2_IRQHandler(void)

{

if(ADC_GetITStatus(ADC1,ADC_IT_JEOC)){ /* Set PC.06 pin */

GPIOB->BRR = GPIO_Pin_11; // reset

GPIOB->BSRR = GPIO_Pin_11;// set

myadc = ADC_GetInjectedConversionValue(ADC1, ADC_InjectedChannel_1);

ADC_ClearITPendingBit(ADC1, ADC_IT_JEOC);

GPIOB->BRR = GPIO_Pin_11; // reset

}

}

But... i want to send the ADC value to my pc using usart connection... so i had some usart function inserted into the adc jeoc event.... 

So after each injection end of conversion... i  send the data via usart at a baudrate of 460800bps... but i want the full 16bit value to be sent to the pc...

Ok said and done,   the adc channel is tied to a 3.3v signal,   on the usart i have 4093 decimal which is corect if i do the math...

But the problem which occures is the increase of adc irq handler runtime from 1us to 100us which is huge

.... and its screwing up my pwm signals ( see screenshot bellow)

The adc channel is reading a shunt resistor which is coming from a 3 phase FET stage for a bldc, so i need feedback to the pc as fast as posible....

For example if i send usart to the pc which  takes 100us... in this time  i will have many pwm pulses where i can have overcurrent fault....

So i need to send feedback as fast as posible when reading the first pwm pulse on each of the 3 phases (or 3 pwm high side signals)

Is it posible to send usart data faster? without afecting the adc irq timing?  Some ideas would be wellcomed

Thanks

[Tesla DeLorean]

Posted on February 04, 2017 at 13:43

Don't put blocking USART code in the IRQ Handler. Put it in a buffer served by the USART TXE IRQ. 

[Bogdan]

Posted on February 04, 2017 at 14:01

Hello Clive

i was thinking of filling a variable inside the adc handler,    and from there  DMA to flush the variable data to the usart tx buffer, and send it out. Is it posible?

[Tesla DeLorean]

Posted on February 04, 2017 at 14:33

Yes, but you'd still need to ensure that the DMA buffer has vacated before starting another, or queue one up. The bandwidth will be limited by the USART baud rate.

[Bogdan]

Posted on February 04, 2017 at 17:02

Ok... so i am guessing either i use DMA or using USART to handle the events.... i will get the same result... but it will not afect the execution speed of the interrupt handlers...

Saying so i had enabled the TXE it from usart and ending with the following code

What do you think of the aproach bellow? ( also listed a logic analyze trace)

TXE IRQ  sets a flag when the data can be sent via USART from main program execution & disables TXE IRQ

After the data is sent,  the TXE IRQ is re-enabled again

void USART1_IRQHandler(void){

if(USART_GetITStatus(USART1,USART_IT_TXE) != RESET){

USART_ClearITPendingBit(USART1, USART_IT_TXE);

GPIOB->BRR = GPIO_Pin_9; // reset

GPIOB->BSRR = GPIO_Pin_9;// set

uflag=1; // set flag for sending data from main program

USART_ITConfig(USART1, USART_IT_TXE, DISABLE); // disable TX interrupt

}

}

void ADC1_2_IRQHandler(void)

{

if(ADC_GetITStatus(ADC1,ADC_IT_JEOC)){ /* Set PC.06 pin */

GPIOB->BRR = GPIO_Pin_11; // reset

GPIOB->BSRR = GPIO_Pin_11;// set

myadcz[1] = ADC_GetInjectedConversionValue(ADC1, ADC_InjectedChannel_1); // store ADC result in variable

ADC_ClearITPendingBit(ADC1, ADC_IT_JEOC);

GPIOB->BRR = GPIO_Pin_11; // reset

}

}

/ *******   main program execution ****************/

int main(void){

while(1) {

if(uflag==1){  // IF flag was set in the USART1 TXE interrupt 

uflag=2;

UART_SendInt0(USART1, myadcz[1]);  // send data

while(USART_GetFlagStatus(USART1, USART_FLAG_TXE) == RESET ); // wait for TX register to empty

UART_SendChar(USART1,'-'); // add a separator between values

while(USART_GetFlagStatus(USART1, USART_FLAG_TXE) == RESET ); // wait for TX register to empty

GPIOB->BRR = GPIO_Pin_9; // reset

USART_ITConfig(USART1,USART_IT_TXE, ENABLE); // enable back the USART TXE interrupt

}

}

}